In a common analysis of monitoring data, survey data obtained at different times are independently inverted to obtain images of each time, and then the images are compared with each other to interpret and monitor the change of foundation along with time, However, in most cases, this approach has high probability of erroneous analysis due to inversion artifacts since the subsurface material properties are not greatly changed.
FIG. 1 is a concept view showing a conventional inversion for imaging geologic structure changing in time.
In the conventional inversion, monitoring data were separately handled to realize subsurface images, so it is impossible to refer to geologic structure changing at different times. Thus, the subsurface images may be seriously distorted, and the distortion appearing in the images of changing geologic structure gets worse. In addition, since each monitoring data and subsurface space are analyzed just in a space concept, it is impossible to consider the change of geologic structure that may happen while measuring the monitoring data.
In order to prevent the above problems, an inversion method for inverting monitoring data by adopting an inversion result of initial data as a prior model or reference model (Locke, 1999; Labrecque and Yang, 2002) has been suggested. However, this method is not different from the conventional inversion in the point that conversion for monitoring data are independently conducted, and also it is apparent that the entire image while the monitoring period is very highly dependent on the initial model.
Meanwhile, the most basic assumption commonly used in the inversion for imaging geologic structure is that the geologic structure is not changed while the data is monitored. However, in case of brine injection experiments on a soil layer having very high permeability or the like, fluid with very high electric conductivity migrates fast, so the above assumption for the static subsurface model can be hardly accepted. In this case, if the factor that the geologic structure changes while obtaining data is not included in the inversion, the subsurface image obtained by the inversion will be severely distorted as apparently understood. In spite of that, most of studies and techniques developed so far are based on the assumption that geologic structure is not changed.
Meanwhile, Day-Lewis et al. (2002) has developed an effective inversion for crosshole radar tomography monitoring data obtained from the geologic structure changing in time while obtaining the data. However, this inversion is applicable only for travel time tomography that uses a travel time of wave, and it is substantially impossible to expand it to inversion of physical survey data.